feat(gradle): rename image to gradle, update FROM, and expand OCI image-spec labels

Convention B (Docker-Hub-style <service>:<full-version>) completes the
gradle+oraclejdk migration. Workflow flips images/tags/subject-name from
gradle-8 to gradle; 8 major-line alias added. Dockerfile FROM now
references the new oraclejdk:8u201 package name from the prior commit;
LABEL maintainer was missing and is added alongside 5 OCI image-spec
keys. Both metadata-action labels: block and Dockerfile LABEL lines
carry the same values so manual and CI builds match.

Sequencing: must land after the prior commit's CI publishes
ghcr.io/tiennm99/oraclejdk:8u201, otherwise the gradle CI build will
fail until oraclejdk re-publishes.

.github/workflows/publish-gradle-8.yml

@@ -40,11 +40,18 @@ jobs:
         uses: docker/metadata-action@9ec57ed1fcdbf14dcef7dfbe97b2010124a938b7
         with:
           images: |
-            ${{ secrets.DOCKER_USERNAME }}/gradle-8
-            ghcr.io/${{ github.actor }}/gradle-8
+            ${{ secrets.DOCKER_USERNAME }}/gradle
+            ghcr.io/${{ github.actor }}/gradle
           tags: |
             type=raw,value=latest
+            type=raw,value=8
             type=raw,value=8.13
+          labels: |
+            org.opencontainers.image.title=Gradle 8
+            org.opencontainers.image.description=Gradle 8.13 on Oracle JDK 8 base
+            org.opencontainers.image.version=8.13
+            org.opencontainers.image.source=https://github.com/gradle/docker-gradle
+            org.opencontainers.image.licenses=Apache-2.0
 
       - name: Build and push Docker images
         id: push
@@ -58,6 +65,6 @@ jobs:
       - name: Generate artifact attestation
         uses: actions/attest-build-provenance@v3
         with:
-          subject-name: ghcr.io/${{ github.actor }}/gradle-8
+          subject-name: ghcr.io/${{ github.actor }}/gradle
           subject-digest: ${{ steps.push.outputs.digest }}
           push-to-registry: true